Teenagers have it easy when it comes to beds.  A twin bed will suffice under most situations.  They sleep by themselves, they are still skinny, and they do not move around a lot while they slumber.  Once you have a sleeping partner, gain some extra weight, or are just looking to set up a nicer bedroom, you will need to consider bed width.  In general, twin, single, and full beds are the most narrow.  Double, queen, and king size beds are equipt with extra space.  If you have the space in your bedroom, go for the biggest bed possible.  You will sleep like a princess.